The first x-ray images were made by exposing photographic film to an area beam of x-ray radiation after it had passed through a patient. Photographic-type film is still the medium of choice for many radiographic procedures, particularly where high image resolution is required. The photographic film may be coupled with a phosphor screen which enhances the film sensitivity to x-ray radiation by converting the x-ray radiation into visible light.
Often, however, it will be necessary for a doctor to view an x-ray image in real-time while performing a procedure such as a cardiac catheterization. In these circumstances, the x-ray film is replaced with an image intensifier and a television camera. The x-ray, striking a phosphor screen at the front of the image intensifier, produces a faint light image which is intensified by the image intensifier and read by the television camera. The use of the image intensifier permits a lower dosage of x-rays commensurate with the need to continuously expose the patient to radiation while a real-time image is acquired.
In certain circumstances it is desirable to convert an x-ray image into a digital representation for processing by a computer. The digital representation of the image may be processed, for example, to enhance edges in the image. Certain techniques such as digital subtraction angiography require that two images, one taken with and one taken without a contrast agent injected into the patient, be subtracted from each other. This subtraction may be done easily with digitized images.
Digital images may be obtained by scanning conventional photographic film or by using a photostimulable phosphor plate which is exposed like film then taken to a reader to be scanned and digitized. Alternatively, the electrical signal output by the television camera on an image intensifier/television camera system may be converted directly to a digital signal through the use of a high speed analog to digital converter.
With improvements in the fabrication techniques for constructing large area integrated circuit arrays (such as are used in LCD-type computer displays) there has been considerable interest in constructing a large area solid state x-ray detector that provides a digital signal directly to processing equipment. One such detector design described in U.S. Pat. No. 4,996,413 issued Feb. 26, 1991 to the same assignee as that of the present invention and hereby incorporated by reference, employs an array of cells comprised of a photodiode and thin film transistor switch arranged in columns and rows beneath a phosphor. An intrinsic capacitance associated with each diode is first charged and then the array is exposed to x-rays. X-ray photons striking the phosphor produce light photons which then strike the photodiodes causing charge to be lost from their intrinsic capacitances. After a period of exposure, charge is restored to the photodiodes. The amount of charge restored to each photodiode indicates the x-ray dose received by each photodiode. An electrical signal indicating the restored charge is digitized and stored as a digital image.
In order to provide suitable spatial resolution, a large number of photodiodes are employed. The wiring necessary to connect each photodiode to the necessary charging and measuring circuitry is reduced by connecting the photodiodes to individually addressable columns and rows. Specifically each photodiode is connected through a solid state switch to a column conductor common to all the other photodiodes in a given column. The photodiodes may therefore share wiring by being read-out one at a time through time division multiplexing. Specifically, a single column conductor provides a charging current to all photodiodes in a given column and is connected to a separate measuring circuit for that column which can quantify the amount of charging current provided to the photodiodes of that column. Control terminals of the solid state switches, which when asserted allow current to flow to the photodiodes, are connected to row conductors common for all the diodes of a given row. Thus, after exposure of the photodiodes, the photodiode array may be scanned by selectively asserting one row conductor to charge all the photodiodes in a given row. Because only one photodiode of that row is connected to each column conductor, the amount of current flowing through the column conductor when a given row conductor is asserted is related to the recharging of a single photodiode. This process is repeated with each row conductor being successively asserted until each of the photodiodes is recharged and the amount of restoring charge required measured.
Attached to each column conductor, so as to measure the charge passing into the column conductor, is an integrator which integrates the current flowing into the column conductor over the time that each row is asserted to produce a total charge measurement. At the end of integration, prior to the charge measurement, the integrator must be allowed to "settle" for a short period of time to remove the effect of noise spikes caused by the switching of the solid state switches coupled to the column conductors by the crossing row and column conductors. After the charge measurement, the integrator must be reset prior to the next row being measured.
For a variety of reasons, it is desirable to reduce the amount of time required to scan the entire panel and acquire the x-ray image. For example, photodiode dark current will deplete the charge stored by the photodiode even in the absence of light. The larger the time between successive scans for a given row, the higher the contribution the dark current will be to the perceived signal. Furthermore, certain x-ray imaging applications require high image rate acquisition, which implies a high row rate and limits the amount of time that can be devoted to the assertion and signal integration of any given row. This time is further reduced by the need to stabilize and reset the integrator as described above.
As a result of the need for high row rates and required stabilization and reset times, the charge on the photodiodes, may not be fully restored. This unrestored charge creates a "ghost image" that is overlaid on the image obtained in the next scanning of the detector array. The magnitude of the ghost image can be reduced by increasing the recharging time during the scanning of each row, but this is undesirable as the row rate would be decreased and limit the frame rate at which images are obtained.